Electrostatic screen printing using a toner repelling screen



Dec. 31, 1968 K. W. RAREY ETAL 3,418,930

ELECTROSTATIC SCREEN PRINTING USING A TONER REPELLING SCREEN Filed Oct.15, 1965 Sheet of 2 MAGNH'UDE 26 I162 CONTROL. 3% 40 H \GH 44 Hvv. "Eg$UPPLY 48 5 E48 go 32 A 2 o i H 3U P PLY 2 INVENTORS 26 KENNETH u).RAREY EN HM BKENNEDQJr.

ATTORNEYS Dec. 31, 968 K. w. RAREY ETAL ELECTROSTATIC SCREEN PRINTINGUSING A TONER REPELLING SCREEN Sheet Filed Oct. 15, 1965 INVENTORSKENNETH m. RAIZEY uouu BKEMHEDYAI. m VJQ, M3, M

"IIIIIIIIIIIIIIIIIAI ATTORNEYS United States Patent 3,418,930ELECTROSTATIC SCREEN PRINTING USING A TONER REPELLING SCREEN Kenneth W.Rarey, South Holland, and John B. Kennedy,

Jr., Chicago, Ill., assignors to Continental Can Compani, Inc., NewYork, N.Y., a corporafion of New Yor Filed Oct. 15, 1965, Ser. No.496,516 Claims. (Cl. 101-114) ABSTRACT OF THE DISCLOSURE The disclosurerelates to electrostatic screen printing wherein a toner-repellingscreen is maintained at a potential which is like that induced upon thetoner, and the toner is carried toward the toner-repelling screen byconductive carrier particles. Upon impact of the carrier particles andtoner with the screen, the toner is dislodged from the carrier particlesand is either repelled from the screen toward the substrate or repelledfrom the screen toward a base electrode.

This invention relates to methods and apparatus for coating a substrateand more specifically to the deposition of printing material upon asubstrate through the forces exerted by electrical potentials. Ifpotentials yielding printing material repulsion forces and/ orattraction forces are selectively applied to the elements of theinvention, the printing material is concentrated upon desired areas(such as a substrate) and its deposition upon undesired elements iseliminated or substantially reduced.

Briefly, a developer mixture including carrier particles having toneradhering thereto by triboelectric forces is caused to be acceleratedtoward a screen, which may have selected open and closed portions fordefining a pattern. Certain of the toner will be dislodged from thecarrier particles with the toner continuing through the open areas ofthe screen while the carrier particles are halted at the screen.Electric fields are provided for accelerating the carrier particles andthe toner and for urging the dislodged toner to deposit upon thesubstrate. A salient feature of the invention relates to the potentialsapplied to the various elements of the process and apparatus.

The art of electrostatic printing is rapidly developing and has achieveda high degree of success. However, numerous problems remain and thepresent invention relates to the solution of one of the problems. It isknown to employ an electrostatic field to effect movement of particlesfor deposit in a pattern upon a substrate, the pattern being previouslydetermined by causing the particles to be arranged upon the surface in apredetermined configuration and then accelerating the particlestherefrom to the substrate. That is, it is known to employ a selectivelycharged substrate so that some areas thereof attract and retainparticles while others do not. Further, it is known to bring theparticles to a charged pattern screen of conductive material whereby theparticles are charged and are then attracted toward a substrate which,if of conductive material, is polarized oppositely to the chargedparticles and, if the substrate is of a non-conductive material, ispositioned in front of a back electrode which is charged oppositely tothe particles. In previous efforts, it was believed that if a potentialwas maintained upon the patterned screen or screen-stencil, thepotential must be one so as to attract the toner particles. However,this procedure introduced additional maintenance problem in that thetoner would build up on the pattern screen or screen-stencil so thatperiodic cleaning of the screen was necessary. This problem is overcomeby causing the patterned screen or screen-stencil to be of likepotential to the toner so that repulsion exists between the toner andthe patterned screen or screen-stencil. In this manner, theaforementioned build-up of the toner on the patterned screen iseliminated or at least substantially and materially reduced.

As taught by the present invention, a mixture of coarse conductivecarrier particles and fine non-conductive toner particles is prepared,wherein one or more toner particles adhere to each carrier particle toform a developer mixture, the adhesion occurring by triboelectriceffects when the carrier particles are mixed with the toner. A baseelectrode is electrically charged and such developer mixture in contacttherewith has the conductive carrier particles charged therefrom so thata repulsion effect is established to cause the developer mixture to berepelled from the base electrode. A pattern or stencil is presented inthe path of the developer mixture and a screen in such path servesmechanically to prevent the passage of the coarse carrier particlestherefrom while allowing the fine toner particles to continue onward toform a deposit upon the substrate in accordance with the pattern orstencil.

When the substrate is of conductive material, it is charged oppositelyto the toner particles for controlling the paths thereof from the screento the substrate. When the substrate is of a non-conductive nature, abacking or back electrode bearing a predetermined potential is placed atthe rear of the substrate and on :a side opposite to the side of thesubstrate facing the screen and base electrode.

The use of the words toner or toner particles is employed herein todesignate particles smaller than the mesh of the screen, capable ofadhesion to the carrier particles by a force less than that developedupon impact of the carrier particles against the screen, and capable offorming a pattern effect on the substrate. In usual practice, the tonerparticles are of a different color than the substrate surface which isto be printed. Toner may be negatively charged or positively charged.Toner particles are charged triboelectrically by mixing them withcarrier particles. The polarity of charge the toner particles acquire isdependent upon the properties of both the toner and carrier material.Preferably, toners are used which when mixed with iron or nickel carrierparticles, exhibit a negative charge. Negatively charged toner particlesare preferably of non-conductive materials and commercially availabletypes are manufactured by the Xerox Corporation. The non-conductivetoner particles comprise essentially pigmented or dyed particles of, forinstance, a mix ture of n-butyl methacrylate (41%) and polystyrene (59%)with an average particle size of about 17 microns. Toner capable ofassuming a charge which is positive with respect to a reference iscommercially available from Interchemical Printing Inks and isdesignated XRL 8969.

Many distinct advantages arise from the use of conductive carriers andnon-conductive toners. Non-conductive toners can be made from materialof low melting temperature. Thus, the materials can generally be fusedto print objects more readily than other toners.

A further important advantage in using a carrier with the toner in anelectrostatic printing apparatus is to promote ease of handling anddistribution of the toner. Whereas toner particles alone are difficultto handle and stick to everything they touch including each other, tonerand carrier flow readily and may be easily distributed uniformly overextended areas. The toner in a developer mixture normally contributesfrom about one to about four percent of the total mass so that theability of the mix ture to flow is due to the presence of the carrier.

Further advantages are evident. The toner particles may acquireconsiderably greater velocities by this method than if they wereaccelerated without a conductive carrier.

The large conductive carrier experiences conductive charging because ofcontact with the base electrode and can attain relatively large terminalvelocities. The small toner particle passes through the apertures in astencil, or other type of screen, with this high velocity so that thetoner particle can travel a great distance to an object to be printed.In addition, the time to travel the given distance can be substantiallyreduced which results in more rapid printing speeds. Further, unlikeconductive toner particles, a non-conductive toner is useful forprinting on conductive surfaces. A conductive particle would surrenderits charge and be easily displaced from such a surface.

Accelerating the toner particles prior to introduction through thestencil, or other type screen, also insures that the velocity vectorassociated with the toner particles is properly oriented, i.e.,perpendicular to the screen stencil at time of passage through thestencil. This insures that minimal dispersion of the pattern will occurduring transport. These two characteristics, high magnitude and properorientation of the velocity vector at the time of introduction throughthe screen, are unique with methods and apparatus exemplified by thepresent invention.

Accordingly, it is the principal object of the present invention toimprove printing and coating processes and apparatus of the typeemploying electrical phenomenon.

It is a further object of the present invention to improve electrostaticprinting and coating processes of the type wherein non-conductive toneris propelled through a pattern defining screen to be deposited upon asubstrate in accordance with the defined screen.

It is a further object of the present invention to provide a means foreliminating or substantially reducing certain maintenance procedurespreviously associated with electrostatic screen process printing andcoating.

It is a further object of the present invention to provide a means foreliminating or substantially reducing electrical shock hazardspreviously associated with electrostatic screen process printing andcoating devices employing high electrical potentials.

It is a still further object of the present invention to provide a meansfor more efiiciently utilizing the electrical potentials established onthe potential bearing elements of electrostatic screen process printingand coating apparatus.

In the present invention, toner and carrier particles becometriboelectrically charged when mixed with the result that the smallertoner particles adhere to the larger carrier particles. The carrierparticles have a diameter several times that or" the toner so that thetoner forms a particulate coating on the carrier particles due to theirelectrical attraction. In one embodiment of the invention, tone-rcapable of assuming a negative charge is employed along with ironfilings, as the carrier particles, which become positively charged. Whenthis developer mixture is employed to print or coat a substrate, thecarrier particles are also charged by contact with a base electrode anda screen. This charging is superimposed upon that due to triboelectricetlects. The carrier particles acquire sufi'icient charge that theelectric field causes them to be repulsed by the base electrode andattracted to the screen. At the screen, the carrier particles lose theircharge and are then repelled away from the screen and attracted to thebase electrode. Thus, the carrier particles, transporting the toner,oscillate back and forth between the base electrode and the screen.These oscillations occur as long as a sufficient potential difference ismaintained between the screen and the base electrode, regardless of theelectrical polarities associated with these components or with thetoner. The carrier particles carry toner particles that are eventuallydislodged by collisions with the screen, the base electrode, or othercarrier particles.

In previous work, it appeared that the screen had to be maintained at apolarity, relative to the base electrode, which was opposite to thecharge carried by the toner.

Subsequent work as shown by the present invention discloses that this isnot true and that many advantages occur if the screen can always repelfree toner, i.e., the screen is maintained at a polarity which is likethe charge carried by the toner.

For example, the screen may be maintained at a negative potential, thebase electrode at a positive potential and the toner negatively chargedwhich is like the negative potential of the screen. In addition, if aback electrode is used, which would be the case for non-conductivesubstrate, the back electrode should be maintained at a positivepolarity. With this arrangement, an electric field is maintained onopposite sides of the screen and oriented in opposite directions. Usingthe conventional means of describing such fields, it is stated that theelectric field intensity vector is directed away from both the baseelectrode and the back electrode and toward the screen. An isolatednegatively charged toner particle would thus experience electricalforces which would tend to move it to- Ward the base electrode or theback electrode depending upon which side of the screen it was located.

Consider a substrate to be coated or printed upon in the followingmanner: First, the base electrode is uniformly coated with atoner-carrier developer mixture. Next, a voltage is applied, observingthe polarities set forth, and of sufiicient magnitude to producevigorous oscillation of the carrier particles. When the voltage is firstapplied, the carrier particles become positively charged. Theseparticles along wih the toner adhering thereto are carried upwardly bythe electric field until they strike the screen. The much smaller tonerparticles, which had formed a discontinuous coating on the carrierparticles are dislodged at this impact. Some of the toner passes throughthe open apertures of the screen, experience an electric force due tothe potential diflerence between the screen and the back electrode, andare directed onto the object to be printed. However, the remaining toneris blocked by the screen, 'by the closed apertures in the screen, or bythe carrier particles. This toner experiences an electric force due tothe potential ditference between the screen and the base electrode andis accelerated back to the base electrode.

A number of advantages accrue to this method of operation in that thescreen does not accumulate toner while printing or coating is occurring.Toner either is deposited upon the substrate, the carrier, or the baseelectrode, the base electrode being relatively easy to maintain in aclean condition. In addition, the base and back electrodes may beoperated at ground potential, with only the screen, which ismechanically shielded by the base and back electrodes, at a highpotential relative to ground. This arrangement provides a convenientsafety feature in that the potentials employed may range upwards ofhundred thousand volts. Also, the toner particle falls through the samepotential diiference twice, effectively doubling the applied voltage.First, it is carried from ground to the screen potential because it isattached to the carrier particle which is oppositely charged. When thecarrier particle is suddently stopped by the screen, the toner particlesthat pass on through the screen presumably do so at high velocity and,ideally, essentially that velocity of the carrier particle just beforeimpact. The toner now falls from the screen potential back to groundwith a further increase in velocity. The toner particle, in effect, canfall through the same potential difference twice, and end up at theinitial potential with an increase in velocity. In etTect, the particlehas changed polarity midway in its fall. Actually, the polarity of thetoner has remained the same but it was carried part way by an oppositelycharged carrier particle.

The invention both as to its organization and method of operationtogether with further objects and advantages thereof will best beunderstood 'by reference to the following specification taken inconjunction with the ac companying drawings in which:

FIGURE 1 is a diagrammatical view illustrating a single developerparticle which is comprised of a larger carrier particle and a pluralityof small toner particles, a plurality of the carrier particle-tonerbeing known as the developer mixture;

FIGURE 2 is a diagrammatical view illustrating the relative potentialsmaintained on the electrodes and the screen, a means for varying thosepotentials and switch means for disconnecting the potentials;

FIGURE 3 is a diagrammatic view of the invention and illustrating acarrier particle with tOner being accelerated toward the screen, tonerafter being dislodged and being attracted to the base electrode, and thecarrier particle, now oppositely charged being accelerated back to thebase electrode;

FIGURE 4 is a diagrammatic view of one embodiment of the invention andillustrating the coating or printing upon a non-conductive substrate;

FIGURE 5 is a diagrammatic view of another embodiment and illustratingthe screen-stencil combination and printing upon a non-conductivesubstrate;

FIGURE 6 is a diagrammatic view of another embodiment of the inventionand illustrating the printing upon a conductive substrate;

FIGURE 7 is a diagrammatic view of another embodiment of the inventionand illustrating the grounding of the screen electrode and the base andback electrodes being maintained at a positive potential relativethereto;

FIGURE 8 is a diagrammatic view of another embodiment of the inventionand showing the base and back electrodes being at ground potential whilethe screen is maintained at a negative potential relative thereto;

FIGURE 9 is a perspective view of a stencil-screen combination which maybe employed in the practice of the invention;

FIGURE 10 is a diagrammatic side elevational view of another form of theinvention and illustrating more a practical application thereof;

FIGURE 11 is a sectional view taken along the line 1111 of the FIGURE10;

FIGURE 12 is a diagrammatic view illustrating the potentials applied tothe electrodes and the screen when employing toner capable of assuming apositive electrostatic charge; and,

FIGURE 13 is a diagrammatic view, similar to the FIGURE 1, butillustrating a single developer particle which is comprised of anegatively charged carrier particle having a plurality of positivelycharged toner particles adhering thereto.

With reference to the FIGURE 1, one particle of the developer mixture isgenerally indicated by the numeral 20 which is comprised of a largecarrier particle 22 and a plurality of toner particles 24 adheringthereto. The carrier particles 22 are formed of a conductive materialsuch as charcoal, metal shot made of various materials such as steel,aluminum, copper or, preferably, small granular iron filings of a rangefrom around 25 to 1000 microns. It will be apparent from the followingdescription that the lower limit of particle size is determined by theapertures in the screen utilized in the present invention since thescreen must not pass the carrier particles. The toner particles 24 arepreferably of non-conductive material and are commercially availablethrough the Xerox Corporation. The composition of the toner has beenpreviously set forth. It is apparent that the carrier particles 22 aremuch larger than the toner particles 24 and as illustrated in the FIGURE1, a number of toner particles 24 can adhere to a single carrierparticle 22 because of well-known triboelectric forces.

A generalized embodiment of the invention is shown in the FIGURE 2wherein a base electrode 26 is in spatial relationship with a backelectrode 28, a conductive screen 30 being positioned therebetween. Ahigh voltage supply 32, which may be of the order of 100,000 volts, iscontrolled by a magnitude control 33 and has its positive outputconductor coupled through a resistor 34 to a switch 36 which, Whenclosed, applies the positive potential to the base electrode 26.Similarly, the positive conductor of the high voltage supply 32 iscoupled through a resistor 38 to a switch 40 and when the switch 40 isclosed, the positive supply is connected to the back electrode 28. Thenegative output from the high voltage supply 32 is connected through aresistor 42 to a switch 44 and when the switch 44 is closed, thenegative potential is applied to the screen 30.

To summarize the showing in the FIGURE 2, the screen 30 is maintained ata potential which is negative with respect to the base electrode 26 andthe back electrode 28. It will be understood that a referencing pointsuch as grounding of certain of the electrodes, can be established andthese embodiments are shown in later figures. Further, the resistors 34,38, and 42 are not necessary but are inserted to act as current limitersin the event of arc-over.

The FIGURE 3 is a view illustrating one carrier-toner of the developermixture 20 being accelerated from the base electrode 26 and anotherreturning to the electrode 26. Positioned immediately above the screen30 is a stencil 46 hearing a plurality of apertures 48, the apertures 48defining the pattern to be impressed upon the back electrode 28, theback electrode 28 in this case being a conductive substrate upon whichthe coating or printing is to be deposited. It will be understood thatthe screen 30- stencil 46 could be combined as shown in a later figureand not of the dual construction as set forth.

With the potentials applied as shown in the FIGURE 3, an electric fieldE is established between the base electrode 26 and the screen 30 andemploying the conventional manner of illustrating such electric fields,the field would be directed in the direction as indicated by the arrow,i.e., toward the screen 30. Also, an electric field E is establishedbetween the back electrode 28 or substrate to be printed upon and thescreen 30 and also oriented toward the screen 30 as indicated by thearrow. After toner 24 is caused to adhere to the carrier particles 22through triboelectric forces such as mixing or agitating the toner 24and carrier particles 22 together, the developer mixture is flowed ontoin any suitable manner the base electrode 26. When the positivepotential is applied as shown in the FIGURE 3, the carrier particle 22becomes positively charged and is accelerated away from the baseelectrode 26. The carrier particles 22 carrying the toner 24 will strikethe screen 30 and if the position of the toner 24 is substantially inline with one of the apertures 48, then that toner will be released fromthe carrier particles 22 and advanced through the screen 30 and aperture48 of the stencil 46. Immediately, the released toner 24 above thescreen 30 experience an electric force to urge them against the backelectrode 28, to be deposited upon the electrode 28 in a patternaccording to the position of the apertures 48 of the stencil 46.

Upon striking the screen 30, the carrier particle 22 now takes on thepotential of the screen 30 (negative in the embodiment set forth in theFIGURE 3) and is immediately repelled by the screen 30 back toward thebase electrode 26. Some of the toner 24 may become dislodged from thecarrier particle 22 or may stay attached to the particle 22 through thetriboelectric force still present. In actual practice, it will beunderstood that a great quantity of developer mixture is suppliedthrough the area above the base electrode 26 so that a cloud is formedand sufficient toner is passed through the screen 30 and apertures 48 ofthe stencil 46 so as to accomplish the coating or printing operation ina minimum of time.

As shown in the FIGURE 4, the embodiment illustrates the printing upon anon-conductive substrate and the positioning of the stencil between thebase electrode 26 and the screen 30. The stencil is identified as 50 andbears a plurality of apertures 48 through which the toner 24 may pass. Anon-conductive substrate 52 such as paper, plastic, etc., is positionedbetween the screen 30 and the back electrode 28 and will receive thetoner 24 as determined by the apertures 48 of the stencil 50. As setforth earlier, in the event that the substrate to be printed upon is ofa conductive nature, then the substrate may be made the electrode, suchas the electrode 28 of the FIGURE 3. As the cloud of developer mixtureoscillates between the base electrode 26 an dthe screen 30-stencii 50,the toner particles 24 may be released and advanced through the screen30-stencil 50 and to the substrate 52 or blocked and returned to thebase electrode 26, with or without a carrier particle 22, to attachthemselves to other carrier particles 22 in the event that they may havebecome dislodged from their earlier carrier particle. Thus, there are nonet forces tending to retain the toner at or near the screen 30 so thatthe build-up of toner on the screen 30 is inhibited.

The embodiment shown in the FIGURE is similar to the embodiment of theFIGURE 4 except that the screen 30-stencil 46 of the FIGURE 3 has beensubstituted for the screen 30-stencil 50. In this embodiment, it will benoted that the stencil 46 is positioned between the sceen 30 and thesubstrate 52, which is the preferred embodiment or manner of deployingthe stencil 46.

The FIGURE 6 illustrates the manner of printing on a conductivesubstrate which is similar to the showing of the FIGURE 3 except for theconnections of the high voltage supply 32. Since the substrate or backelectrode 28 of the FIGURE 6 is conductive, it may be made the electrodefor establishing the electric field E and the substrate 52 will receivethe coating or printing of toner as defined by the apertures 48 of thestencil 46.

In the FIGURE 7, the screen 30 has been placed at ground potential asshown by the conductor 54 so that the base electrode 26 and the backelectrode 28 are now the prescribed volts above ground. In certainenvironment, it may be determined that the screen 30 be maintained atground potential.

Alternatively, in the FIGURE 8, the conductors of the power supply 32coupled to the base electrode 26 and the back electrode 28 is groundedby a conductor 56 so that the screen 30 is now maintained at theprescribed volts below ground. The embodiment of the FIGURE 8 introducesa safety feature in that the outside electrodes, those electrodes 26 and28 which may be or come in contact with an operator, are maintained atthe harmless ground potential whereas the inner electrode or screen 30bears a high negative charge and the screen 30 may be effectivelyinsulated and maintained away from an operator.

The FIGURE 9 illustrates a stencil-screen combination 58 which may be ofa conductive material and wherein the pattern or characters A, B and Chave been formed. If the stencil-screen 58 is employed, then it wouldreplace the screen 30-stencil 46 of the earlier figures. In the earlierfigures, the stencil 46 is of a normally nonconducting material. If thestencil-screen 58 of the FIG- URE 9 is utilized, then any suitablepattern or indicia may be formed thereon and in the specificstencil-screen 58 illustrated, images of the first three alphabeticalcharacters would be transferred to either the back electrode 28 or thesubstrate 52.

In the FIGURE 10, a practical embodiment of the invention is illustratedwherein a funnel-shaped receiver 60 would be filled with carrierparticles 22 and toner 24 and allowed to mix together and drop upon aninclined base electrode 26'. Upon the mixing or agitation of the carrierparticles 22 with the toner 24, the developer mixture would be formeddue to the triboelectric forces as previously set forth. The inclinedbase electrode 26' is maintained in its position as shown by a stand 62which, for convenience, may be of an insulating material since apositive potential is applied to the electrode 26' by the high voltagepower supply 32. A negative potential is applied by the power supply 32to the inclined screen 30 while a potential identical to the potentialupon the base electrode 26' is applied to an inclined back electrode28'. A substrate 52' is positioned between the inclined stencil 46 andfor convenience, means (not shown) may be employed to advance thesubstrate 52' with respect to the back electrode 28. A pattern or imagewill be formed upon the substrate 52' in accordance with the apertures48 of the stencil 46 by the oscillating developer mixture 20 whichcarries the toner 24 through the screen 30' and apertures 48. A tray 64is provided to receive the carrier particles 22 and/or toner 24 as theyemerge from the lowemost portion of the back electrode 26.

The FIGURE 11 is a sectional view taken along the line 1111 of theFIGURE 10 and is of primary interest in that it discloses a plurality ofnon-conducting or insulating spacers 66 which are employed to maintainthe base electrode 26, screen 30-stenci1 46', and substrate 52-backelectrode 28' in spatial relationship. It will be understood thatrollers could be substituted for the upper spacers 66 so that thesubstrate 52 could be rolled or conveniently advanced past the printingstation of the FIGURES 10 and 11.

In the event that toner which adheres to a carrier through triboelectricforces wherein the toner takes on a positive charge, is employed, thenthe embodiment shown in FIGURE 12 would be suitable for causing coatingor printing upon the substrate 52. The apparatus of the FIGURE 12 issubstantially identical to the apparatus of the FIGURE 8 except that thescreen is maintained at a potential which is positive with respect toground. Such positively charged toner is shown in the FIGURE 13 andidentified as positive developer mixture 68. The apparatus of the FIGURE12 would operate in a manner similar to the foregoing figures, i.e.,oscillation of the developer mixture 68 would occur between the baseelectrode 26 and the screen 30 and certain of the toner particles wouldbe dislodged so as to advance through the apertures 48 of the stencil 46and be deposited upon the substrate 52. Since the polarity of theelectrodes 26, 28 and the screen 30 have been reversed, the electricfield would now be oriented in a direction opposite to that shown in theFIGURE 3.

Accordingly, the present invention has described and illustrated amethod and apparatus which has overcome a number of problems inherentwith electrostatic printing devices heretofore known. In the presentinvention, the center or screen electrode does not accumulate tonerwhile printing is occurring since the electric fields are so orientedthat the toner either ends up on the substrate, the carrier particle, orback at the base electrode. In addition, the base and back electrodesmay be operated at ground potential with only the screen, which ismechanically shielded by the two electrodes, at a high potentialrelative to ground. This provides a convenient safety feature. Also, thetoner particle falls through the same potential difference twice,effectively doubling the applied voltage. In effect, the particle haschanged polarity midway in its fall. Actually, the polarity of the tonerhas remained the same but it was carried half way to its target by anoppositely charged carrier particle.

After the toner has been deposited upon the substrate or electrode inthe desired pattern, it will be readily understood that the toner may befixed in any suitable manner such as by heating, solvent action, etc. Inaddition, positively charged as well as negatively charged toner may beemployed in the practice of the invention although the greater emphasishas been on the illustration of the toner which takes on a negativecharge when mixed or agitated with a particular carrier particle.

Thus, the present invention may be embodied in other specific form-swithout departing from the spirit and the essential characteristics ofthe invention. The present embodiment is, therefore, to be considered inall respects as illustrative and the scope of the invention beingindicated by the appended claims rather than the foregoing description,and all changes which come within the meaning and range of theequivalency of the claims are, therefore, intended to be embracedtherein.

What is claimed is:

1. Apparatus for coating a substrate comprising a base electrode inspaced relation to the substrate, means defining a pattern to be coatedupon the substrate and positioned between the substrate and said baseelectrode, carrier particles having toner of a predetermined electriccharge adhering thereto by triboelectric forces, means for acceleratingsaid particles toward said means defining a pattern, and means forestablishing an electric field between the substrate and the meansdefining a pattern, the predetermined electric charge of the toner beingof like polarity to the polarity induced upon the means defining apattern by said means for establishing an electric field and of unlikepolarity to said base electrode.

2. Apparatus for coating a substrate comprising a base electrode inspaced relation to the substrate, a screen positioned between said baseand the substrate, a stencil for defining a pattern to be coated uponthe substrate and positioned between said screen and the substrate,carrier particles having toner of a predetermined electric chargeadhering thereto by triboelectric forces, means for accelerating saidparticles from said base electrode through said screen to the substrate,and means for establishing an electric field between the substrate andsaid screen, the predetermined electric charge of the toner being oflike polarity to the polarity induced upon the screen by said means forestablishing an electric field and of unlike polarity to said baseelectrode.

3. Apparatus for coating a substrate comprising a base electrode and aback electrode in spaced relation so as to permit the positioning of thesubstrate therebetween, means defining a pattern to be coated upon asubstrate and positioned between the substrate and said base electrode,carrier particles having toner of a predetermined electric chargeadhering thereto by triboelectric forces, means for accelerating saidparticles from said base electrode toward said means defining a pattern,and means for establishing an electric field between said back electrodeand the means defining a pattern so as to induce to be deposited uponthe substrate as determined by said means defining a pattern, thepredetermined electric charge of the toner being of like polarity to thepolarity induced upon the :means defining a pattern by said means forestablishing an electric field and of unlike polarity to said base andback electrodes.

4. Apparatus for coating a substrate comprising a base electrode inspaced relation to the substrate, means defining a pattern to be coatedupon the substrate and positioned between the substrate and said baseelectrode, carrier particles having toner of a predetermined electriccharge adhering thereto by triboelectric forces, means for establishinga first electric field between said base electrode and said meansdefining a pattern and directed toward said means defining a pattern soas to accelerate said particles toward said means defining a pattern,and means for establishing a second electric field between the substrateand the means defining a pattern and directed toward the means defininga pattern so as to induce toner to deposit upon the substrate, thepredetermined electric charge of the toner being of like polarity to thepolarity induced upon the means defining a pattern by said means forestablishing an electric field and of unlike polarity to said baseelectrode.

5. Apparatus for coating a substrate comprising a base electrode inspaced relation to a back electrode and permitting the positioning of asubstrate therebetween, a screen electrode positioned between thesubstrate and said base electrode, carrier particles having toner of apredetermined electric charge adhering thereto by triboelectric forces,a power supply, means for coupling said power supply to said screenelectrode and said base electrode for causing acceleration of saidparticles away from said base electrode toward said screen electrode,

and means for coupling said power supply to said screen electrode andsaid back electrode for establishing an electric field between saidscreen electrode and base electrode and directed away from said backelectrode so as to induce toner to be deposited upon the substrate, thepredetermined electric charge of the toner being of like polarity to thepolarity of the screen and of unlike polarity to said base electrode andback electrode.

6. The combination as defined in claim 5 including a stencil fordefining a pattern to be coated upon the substrate and positionedbetween the substrate and said screen electrode.

7. The combination as defined in claim 5 including a stencil fordefining a pattern to be coated upon the substrate and positionedbetween said screen electrode and said base electrode.

8. Apparatus for coating a substrate comprising a base electrode inspaced relation to the substrate, a screen positioned between said baseelectrode and the substrate, carrier particles having negatively chargedtoner adhering thereto by triboelectric forces, means for providing apositive electrical potential on said base electrode and the substrateand means for providing a negative electrical potential on said screen,the potentials so provided establishing a first electrical field betweensaid screen and said base electrode and oriented toward said screen soas to propel the positively charged carrier particles supporting thenegatively charged toner toward said screen and a second electricalfield between said substrate and said screen and oriented toward saidscreen so as to urge the toner toward the substrate to be depositedthereon, the carrier particles being sufficiently large enough to beprecluded from passage through said screen so as to oscillate betweensaid screen and said base electrode.

9. The combination as defined in claim 8 including individual meanscoupled to each of said base electrode, screen, and substrate forvarying the magnitude of the potential applied thereto.

10. A method for coating a substrate comprising the steps of positioninga base electrode and a substrate in spatial relationship, providing ascreen between the base electrode and the substrate, supporting carrierparticles retaining negatively charged toner thereon upon said baseelectrode, and applying a first electric field oriented toward saidscreen and a second electric field also oriented toward said screen buton the opposite side thereof so as to propel said particles toward saidscreen and to dislodge the toner for deposition upon the substrate.

11. A method for coating a substrate comprising the steps of positioninga base electrode and a back electrode in spaced relation, positioning asubstrate to receive the coating between the electrodes, providing ascreen between the base electrode and the substrate, supporting carrierparticles retaining negatively charged toner thereon upon the baseelectrode, and applying a first electric field oriented toward saidscreen so as to propel said particles toward said screen and to dislodgethe toner and a second electric field also oriented toward said screenbut on the opposite side thereof for urging the dislodged toner todeposit upon the substrate.

12. The method as defined in claim 11 including the step of positioninga stencil for defining the pattern to be coated upon the substratebetween the substrate and said screen.

13. The method as defined in claim 11 including the step of positioninga stencil defining the pattern to be coated upon the substrate betweensaid base electrode and said screen.

14. A method for coating a substrate comprising the steps of positioninga base electrode and a back electrode in spatial relationship, insertinga substrate therebetween for receiving the coating, providing a screendefining a pattern to be coated upon the substrate between the substrateand the base electrode, supporting carrier particles retainingnegatively charged toner thereon upon said base electrode, placing thebase electrode and the back electrode at ground potential, and applyinga potential to the screen which is negative with respect to the base andback electrodes so as to produce a first electric field oriented fromthe base electrode toward the screen so as to propel the particlestoward the screen for dislodging the toner for passage therethrough anda second electric field directed from the back electrode toward saidscreen for urging the dislodged toner from the screen to the substrateto be deposited thereon according to the defined pattern.

15. Apparatus for coating a substrate comprising a base electrodedisposed in spaced relation from a back electrode, stencil-screen meansdisposed in spaced relation between said base electrode and said backelectrode with the space between said stencil-screen means and said backelectrode being sufficiently large enough to accommodate said substratetherebetween, power means for establishing a potential differencebetween said base electrode and said stencil-screen means and betweensaid stencil-screen means and said back electrode with the polarity ofsaid stencil-screen means being opposite from the polarity of said baseelectrode and said back electrode, means for maintaining said baseelectrode and said back electrode at ground potential, carrier particlesand toner triboelectrically adhering thereto with the polarity of thetoner being of the same polarity as the stencil-screen means, means fordepositing said carrier particles and toner on said base electrodewhereby the electric field created by the potential difference betweenthe stencil-screen means and said base electrode propels said carrierparticles and toner toward said stencil-screen means, said carrierparticles oscillating between said stencil-screen means and said baseelectrode and being inhibited from passage through said stencil-screenmeans while permitting the propulsion of the toner therethrough, and thetoner being repelled from said stencilscreen means because of their likepolarity.

References Cited UNITED STATES PATENTS 2,940,864 6/1960 Watson.3,081,698 3/1963 Childress et al. 3,285,167 11/1966 Childress et al.

ROBERT E. PULFREY, Primary Examiner.

EDGAR S. BURR, Assistant Examiner.

